It was based on the number of stars/forks and the size of the repository.
There must be something else or something wrong, because you indexed one of my small repo (~100 stars, ~20 forks, ~20Mb) and not the bigger ones (~500 stars, ~100/150 forks, ~150Mb)
Maybe he is limiting it to repositories of 50 MB or less, for example.
or possibly there's a "time decay" element where more recently "popular" repos are prioritized, not just based on absolute start/fork count
Looking around at repositories I'm familiar with this seems to be the case.